1. Field of the Invention
The present invention relates to catalysts uniformly distributed in mesoporous aerogels and ambigels having a three-dimensional nanoarchitecture and a metal-oxide framework. Specifically disclosed are Au—TiO2 aerogels and their oxidation of carbon monoxide. The gold is inserted into the aerogel as a monolayer-protected cluster (MPC). Much of this invention was disclosed in Rolison et al., Nano Letters, 2002, 2(5), 545–549, and Rolison et al., Science, 2003, in press which is incorporated herein by reference including the references cited therein.
2. Description of Background Art
An extensive literature describes the ability of nanometer-sized gold supported on titania (Au/TiO2) to catalyze the low-temperature oxidation of carbon monoxide. The mechanism of oxidation—in particular the individual roles of gold and titania—is still being elucidated. Agreement exists that oxygen is activated at either titania or the gold-titania interface, while carbon monoxide undergoes weak reversible adsorption at the surface of the gold nanoparticle. The surface diffusion of adsorbed CO on the gold to the activated oxygen sites at the interface, with possible spillover of activated oxygen to the gold, enhances oxidative turnover.
Studies by Haruta and coworkers, Catal. Lett. 1997, 44, 83–87 incorporated herein by reference, show that 3-nm gold particles supported on titania optimize the effectiveness of catalytic oxidation of carbon monoxide. Gold sized at 3 nm appears to balance the need for a large ratio of perimeter to bulk, (i.e., high catalyst dispersion) while still presenting enough surface area for carbon monoxide adsorption and surface diffusion. Smaller particles are thought to be partially buried in the titania support, while larger particles are thought to lose carbon monoxide to desorption before diffusion to the active perimeter permits oxidation. Although 12-nm Au supported on iron oxide remains active for CO oxidation, which argues for a less size-dependent mechanism, such as spillover involving reactive atomic oxygen, titania-supported catalysts with gold particles larger than 5-nm are reported to be less active for CO oxidation.